The present invention relates to the vibration resistance properties of optical disk devices, and more particularly relates to the technology of preventing the focus and the tracking of the pickup from being displaced by vibrations applied from the outside.
To reproduce a high quality signal in an optical disk device, the laser spot is ideally always kept in perfect focus and precisely on track with respect to the signal recording surface. However, vibrations of a certain size may occur in the focus direction perpendicular to the signal recording surface or in the tracking direction perpendicular to the direction in which the track is advanced, due to factors such as the mechanical precision of the disk rotating mechanism, the planar precision of the disk itself, or external disturbances. For this reason, optical disk devices, in general, are provided with a focus actuator that drives the objective lens of the pickup in the focus direction and a tracking actuator that drives the objective lens in the tracking direction. These actuators apply the focus servo and the tracking servo to the pickup in order to keep the laser spot always in perfect focus and precisely on track even if the above vibrations occur.
FIG. 4 shows the configuration of a conventional optical disk device. The configuration and the above the control of the conventional optical disk device are briefly described below with reference to FIG. 4.
A disk 1 is an optical disk on which information has been recorded in a predetermined track arrangement. A rotation means 2 is a motor for rotating the disk 1. A pickup 3 focuses a laser beam onto the information recording surface of the disk 1 to form a laser spot, and reads the recorded information by detecting the light reflected therefrom and detects relative position errors in the focus direction and relative position errors in the tracking direction with respect to the information recording surface.
A focus actuator 4 is driven by a focus drive means 5, and moves the pickup 3 in the focus direction. Similarly, a tracking actuator 6 is driven by a tracking drive means 7, and moves the pickup 3 in the tracking direction.
A focus error signal generation means 8 generates a focus error signal FE based on the relative position error detected by the pickup 3. Also, a focus control means 9 controls the focus of the pickup 3 based on the focus error signal FE. Similarly, a tracking error signal generation means 10 generates a tracking error signal TE based on the relative position error detected by the pickup 3, and a tracking control means 11 controls the tracking of the pickup 3 based on the tracking error signal TE. Thus, the laser spot is controlled so that it is always in perfect focus and precisely on track.
On the other hand, when optical disk devices are used in portable applications, for example, they are subject to relatively strong vibrations and impacts from external sources. When these vibrations or impacts exceed the tolerance range of the above control, the laser spot can no longer be maintained in the optimal condition. Accordingly, optical disk devices generally utilize the generation of tracking error signal TE that are caused by external vibrations or the like in order to detect the vibration that is acting on the device, and based on this detection, increase the gain of the focus servo and tracking servo so that the servos are not displaced. Servo controls based on this vibration detection are executed by a bandpass filer 12, a window comparator 13, and a filter control means 14 in the optical disk device shown in FIG. 4. Hereinafter, this control is described briefly.
The bandpass filter 12 is set to pass only the signal component of predetermined frequencies in order to obtain signals that are caused by vibrations. Thus, the tracking error signal TE is passed through the bandpass filter 12, so that particular frequency components are extracted therefrom and output as the vibration component VC.
A predetermined threshold level is set in the window comparator 13, and when the vibration component VC that is input exceeds this threshold level, that is, when the vibration exceeds the tolerance range, the window comparator 13 detects that a vibration is acting on the device and outputs a vibration detection signal VD.
The filter control means 14 receives the vibration detection signal VD and as a result increases the gain of the digital filters that are provided in the focus control means 9 and the tracking control means 11, so as to change the filter properties of these digital filters. Thus, the focus servo and the tracking servo are kept from moving easily, and this makes it difficult for a strong vibration from the outside to displace the focus and the tracking of the pickup.
In general, optical disk devices are more sensitive to, and affected by, vibrations in the tracking direction than vibrations in the focus direction. Consequently, like in the conventional optical disk device shown in FIG. 4, by adopting a configuration that increases resistance to vibrations in the tracking direction, the vibration resistance properties of the device can be secured. That is, conventionally, the vibration resistance properties of optical disk devices were ensured without any particular consideration being given to vibration in the focus direction.
However, when optical disk devices are used in portable applications, for example, they are subject to particularly strong vibrations and impacts in the focus direction. In conventional optical disk devices, this disrupted the focus servo and in the worst cases even displaced the servo.
Additionally, it is expected that future increases in the density of information recorded to optical disks will be accompanied by increasingly shorter focal lengths of the light beam for reading this high density information. When the focal length of the light beam is short, even minor vibrations in the focus direction, which heretofore could be ignored, come to have a significant impact on signal reproduction. Consequently, it is difficult to ensure sufficient vibration resistance properties in conventional optical disk devices that detect vibrations based on only the tracking error signal.